ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-13-675-2013Assessment of the Level-3 MODIS daily aerosol optical depth in the context of surface solar radiation and numerical weather modelingRuiz-AriasJ. A.123DudhiaJ.3GueymardC. A.4Pozo-VázquezD.121Solar Radiation and Atmosphere Modeling Group, Physics Department, University of Jaén, Jaén, Spain2Center of Advanced Studies in Energy and Environment, University of Jaén, Jaén, Spain3Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, Colorado, USA4Solar Consulting Services, Colebrook, New Hampshire, USA18012013132675692This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/13/675/2013/acp-13-675-2013.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/675/2013/acp-13-675-2013.pdf

The daily Level-3 MODIS aerosol optical depth (AOD) product is a global daily
spatial aggregation of the Level-2 MODIS AOD (10-km spatial resolution) into
a regular grid with a resolution of 1° × 1°. It offers
interesting characteristics for surface solar radiation and numerical weather
modeling applications. However, most of the validation efforts so far have
focused on Level-2 products and only rarely on Level 3. In this contribution,
we compare the Level-3 Collection 5.1 MODIS AOD dataset from the Terra
satellite available since 2000 against observed daily AOD values at 550 nm
from more than 500 AERONET ground stations around the globe. Overall, the
mean error of the dataset is 0.03 (17%, relative to the mean
ground-observed AOD), with a root mean square error of 0.14 (73%, relative
to the same), but these errors are also found highly dependent on
geographical region. We propose new functions for the expected error of the
Level-3 AOD, as well as for both its mean error and its standard deviation.
Additionally, we investigate the role of pixel count vis-à-vis the
reliability of the AOD estimates, and also explore to what extent the spatial
aggregation from Level 2 to Level 3 influences the total uncertainty in the
Level-3 AOD. Finally, we use a radiative transfer model to investigate how
the Level-3 AOD uncertainty propagates into the calculated direct normal and
global horizontal irradiances.